A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Two basic requirements for a lithographic apparatus are to image the desired pattern on the substrate as intended and to position the desired pattern within a certain accuracy with respect to a previous patterned layer on the substrate. The last requirement is referred to as “overlay”.
Generally an overlay error is determined by measuring an overlay marker structure positioned at a certain location on a substrate. The overlay marker structure comprises an overlay target in a first layer and an overlay target in a second layer positioned above the first layer. The overlay is determined by identifying a differential position i.e. a difference in position between the overlay target in the first layer and the overlay target in the second layer. However, several problems with this type of overlay determination arise.
Generally, overlay targets in a layer are manufactured in parallel with the pattern structures of that layer. However, during processing, several process steps may deteriorate the overlay targets. Unfortunately, this deterioration often leads to an asymmetric deformation of an overlay marker structure of the prior art. The asymmetry causes an inaccurate determination of the overlay. Especially in future lithographic applications, this decrease in accuracy may have dramatic consequences. Because, as the dimensions of the pattern structures become smaller and smaller, the requirements for parameters like overlay increase accordingly.
Another problem with the determination of overlay lies in the dimensions of a state-of-the-art overlay marker structure. A prior art overlay marker structure, as shown in FIG. 2, comprises features with dimensions several times larger than the exposure wavelength of the lithographic apparatus, i.e. the features are not “at resolution”.
Furthermore, a prior art overlay marker structure generally comprises only bar-like structures in the overlay targets of both layers. It is however well-known that different structures will be affected differently by the optics provided in a lithographic apparatus. For instance, a well-known combination in the art is a device layer comprising bar-like structures called gates and a contact layer comprising apertures called contact holes. Effects of aberrations and distortions, induced by optical elements like projection lenses and/or mirrors, are different for contact holes and bars. As a result, if both layers include bar shaped overlay targets, the overlay marker structure may reveal a perfect alignment. However, pattern structures on the layers may include gates and contact-holes that are not well-aligned when a contact layer is positioned on top of a device layer. Thus, the measured overlay on the overlay target will be different from the actual overlay between the pattern structures.
Finally, in lithographic processing, it is desirable to be able to determine an overlay error as early as possible, and moreover, to compensate for misalignment in the process at an early stage. However, an overlay error between two layers is generally determined off line, i.e. in a separate apparatus not directly connected to the production line of an integrated circuit, by employing some kind of metrology technique. Moreover, often the measurements are performed after the processing of a number of substrates. Other problems exist.